Preserve coroutine Job in generated async bridge R8 rules

[piercifani]

Problem

Since Skip 1.9.13, generated async bridge JNI lookups include Lkotlinx/coroutines/Job; in the method signature. In a minified app, R8 can rename that interface while the native signature remains unchanged. The lookup then fails, and the generated force unwrap crashes. Debug builds without minification do not expose the problem.

Change

Add -keep interface kotlinx.coroutines.Job to the shared defaultProguardContents generator used for app and framework ProGuard files. Extend the native-app initialization test to check the emitted rule.

The manual upgrade guidance is added directly to the existing ProGuard documentation in skiptools/skip.dev#26.

Related: skiptools/skip-bridge#85 (diagnostics for failed bridge lookups).

Validation

• swift test --filter SkipCommandTests: 21 tests passed.
• Standalone local JVM/JNI reproduction using R8 8.10.9-dev and kotlinx-coroutines 1.10.2, with rules extracted from the generator: without the new rule, Job mapped to kotlinx.coroutines.a and GetMethodID(..., "callback_fetch", "()Lkotlinx/coroutines/Job;") failed with NoSuchMethodError; with the rule, Job retained its name and the lookup succeeded. This exercised a real JNI lookup against R8 classfile output, not an Android-device end-to-end test.
• git diff --check passed.

[marcprux]

Ack! Proguard/R8 strikes again.

[marcprux]

I wonder if the skip export command could someday auto-generate the right proguard-rules.pro (or an ancillary proguard-rules-skip.pro) file based on analysis of what functions/types need to be preserved for accessing through skip-bridge. That would have the potential to also drastically reduce the surface area of the skip.** packages that we already keep en masse (and thereby considerably reduce the overall app size).